ABSTRACT

The trachytic Campanian Ignimbrite, which originally covered
an area of 30,000 km2 around Naples, Italy, is the product of a highly
energetic, gas-rich eruption. The deposit lies in valleys and isolated
watersheds. In its medial and distal extent to the south, north
and east of Naples, the Campanian pyroclastic current encountered
mountains exceeding 1000 meters. Anisotropy of magnetic
susceptibility (AMS) measurements indicate that the Campanian
pyroclastic current (the transport system) traveled radially outward
from the Phlegrean Fields area, but the ignimbrite-forming flow (the
deposition system that developed from the base of the transport
system) moved downslope from mountainsides to valleys, including
slopes facing the eruption source, and flowed down drainage systems
from intermontane basins. The Campanian pyroclastic current flowed
~35 km over the water of the Bay of Naples to deposit >43 m of
ignimbrite on the south shore and also overtopped a 685-1000 m high
ridge of the Sorrento Peninsula to deposit more on the other side.
The distribution of the ignimbrite and the measured flow directions
suggest that the Campanian pyroclastic current moved across the
landscape as an expanded (and therefore turbulent) decompressing flow
rather than as a high density, nonturbulent sheet-like current moving
over mountains by momentum acquired by eruption column collapse.
Strong expansion is corroborated by shard morphology that indicates
derivation from highly inflated pumice and suggests vesicles must have
been at least 80% by volume of the original magma.

GENERAL BACKGROUND STATEMENT
The runout length of pyroclastic flows and their ability to surmount topographic
barriers are topics of continuing research germane to the distribution of ignimbrite
sheets. Pumice-rich pyroclastic flows are known to have crossed topographic barriers
of considerable height (Yokoyama, 1974; Miller and Smith, 1977; Koch and McLean,
1975; Rose et al., 1979). The 22,000 yr B.P. Ito pyroclastic flow (Japan) traveled
70 km over topographic barriers as high as 600 m (Aramaki and Ui, 1966; Yokoyama,
1974). The 18,000 B.P. Taupo Ignimbrite, only ~30 km3 in volume, is spread out
over a ~20,000 km2 area and mantles mountains as high as 1500 m above the inferred
vent as far as 45 km from the source (Wilson, 1985; Wilson and Walker, 1985).
Pyroclastic flows from Aniakchak and Fisher calderas in the Aleutian Islands traveled
as far as 50 km over mountainous barriers between 250 and 500 m high (Miller and
Smith, 1977).

Currently, there are two general models that describe the way
that pyroclastic currents move across the landscape -- (1) as expanded
flows (EFs) thicker than the height of the mountains they traverse, or
(2) as dense flows (DFs) moving as a nonturbulent ground-hugging sheet
across the landscape (Sparks, 1976). The purpose of the present paper
is to test these ideas by analyzing the stratigraphy and flow
directions, as determined by anisotropy of magnetic susceptibility
(AMS) measurements (see below), of the Campanian Tuff.

The EF model was introduced first and applied to the Ito
pyroclastic flow, Japan (Aramaki and Ui, 1966; Yokoyama, 1974). The
DF model was applied by Miller and Smith (1977) to pyroclastic
currents that moved outward from Fisher and Aniakchak calderas,
Alaska. This latter model was given quantitative credence by Sparks
et al. (1978) who concluded that a pyroclastic flow develops around
the base of a collapsing eruption column, deflates, and then moves
outward across the landscape under its own momentum. In their model,
the momentum that a pyroclastic flow acquires is proportional to the
height from which the eruption column collapses.

The two models require fundamentally different hydrodynamic
behaviors. For EFs, the pyroclastic current must remain turbulent to
maintain its expansion to a thickness greater than the topography that
it overtops. Also, an expanded current can travel across water
because expansion reduces its bulk density so that only its basal part
interacts with the water (Sigurdsson et al., 1991). Dense pyroclastic
flows probably cannot easily travel above water. Moreover, should
they enter and travel beneath water, viscous boundary effects, mixing
with water, and other conditions would inhibit flow, and it is
unlikely that they could re-emerge.

One critical observation applicable to the problem discussed
herein is that pyroclastic currents such as nuees ardentes are known to
separate gravitationally into a lower part containing most of the
solid fragmental mass. This natural density stratification in
initially turbulent pyroclastic currents (Valentine, 1987) and other
sediment gravity flows (Fisher, 1983, 1984) commonly results in flow
transformations from turbulent to nonturbulent behavior in basal
zones where concentration values become high. Having different
densities and turbulent behaviors, the different parts of the current
can decouple and travel different paths, depositing material
independently (Fisher, 1990). In mountainous terrain, flow
transformations, decoupling and divergent flow directions are
especially amplified. Study of these effects can contribute to a
better understanding of pyroclastic flow emplacement processes.

Sparks et al. (1978) postulated that DFs quickly originate
following fall-back of a turbulent, collapsing eruption column and
then move outward as a non-turbulent flow. Their calculations, using
flow velocities ranging from 10 to 200 m/s, a drag coefficient of
0.01, and terminal velocity measurements of pyroclastic particles by
Walker (1971), showed that grains >1 mm could not be carried in
suspension.

The conclusion that momentum was the main cause of transport of
pyroclastic flows influenced the "energy line" concept of Sheridan
(1979). He explained that the slope of the energy line as proposed by
Hsu (1975) for avalanche runout, traces the potential flow head from
the top of the gas-thrust region of an eruption column to the distal
toe of a flow along the line of transport. McEwen and Malin (1989),
however, argued that the energy-line model predicts velocities that
are too high, resulting in flow paths that are insufficiently
responsive to topography. They suggest that velocity-dependent
resistance factors such as Bingham or turbulent models are needed for
accurate velocity predictions. We further suggest that in
mountainous regions, flow transformations, density stratification,
decoupling and blocking which are discussed below need to be
considered as resistance factors that affect the forward progress of
flows. Fisher (1990) shows that mountainous terrain itself can be
considered as a roughness element that significantly effects runout
distance.

Valentine (1987) concluded that pyroclastic flows may become
density stratified and do not necessarily completely collapse to a
non-turbulent condition of flow. According to his model, density
stratification does not necessarily form a surface above which is
mostly gas and below which is a dense flow, but rather there is a
continuous gradation from one to the other. At flow velocities of
100 m/s or more and 300 m/s or more, particles as large as 1 cm and 10 cm
repectively can be turbulently supported -- considerably larger than
sizes calculated by Sparks et al. (1978). The differences in
supportable clast sizes stem from the choices of substrate roughness
and boundary layer thickness. Valentine (1987) proposed a rougher
terrain than Sparks et al. (1978), with roughness elements (such as
tree stumps) up to 1 m. Sparks et al. (1978) assumed a flat terrain
with a roughness of 1 cm and considered the whole flow as a boundary
layer.

The EF model contends that a pyroclastic current may initially be
of medium- to low-density, but unlike the DF model, it remains
expanded as it travels over the landscape leaving behind a
depositional carpet deposited from its basal part, a model proposed by
Fisher (1966) and extended by Branney and Kokelaar (1992). Flow of
DFs is based upon a plug flow model whereby deposition is thought to
occur by en masse freezing of the debris, similar to nonvolcanic
debris flows, rather than layer by layer accretion (Sparks, 1976).

DESCRIPTION OF THE CAMPANIAN IGNIMBRITE

The Campanian Ignimbrite is a trachytic tuff that crops out
regionally around Naples, Italy, and occurs in isolated
watersheds and valleys. In its medial and distal extent, to the
south, north and east of Naples, the Campanian flow encountered
mountains 1000 meters and more in height. In the proximal area near
the Phlegrean Fields, identification of rocks that belong to the
Campanian Ignimbrite eruption is controversial. Rosi et al. (1983)
and Rosi and Sbrana (1987) hold that its source is in the Phlegrean
Fields, possibly in the Bay of Pozzuoli. Di Girolamo et al. (1984)
and Scandone et al. (1991), however, propose a source north of Naples
beneath the Volturno Plain. Our data from anisotropy of magnetic
susceptibility (AMS) (discussed below) suggests a source within the
Phlegrean Fields area, therefore we use the center of Pozzuoli Bay as
a reference point.

Recently obtained high precision dates using single-crystal,
laser-fusion 40Ar/39Ar methods from samples of the Campanian
Ignimbrite near Avellino indicates an age of 35.5 +-0.8 ka (n=18), and
one from near Maddaloni indicates an age of 36.0 +- 0.6 ka (n=21)(Deino
et al. 1992). But other than these two dates from distal Campanian
Ignimbrite localities, the age of the Campanian Ignimbrite is poorly
constrained. Paterne et al. (1988) dated five trachytic marine tephra
layers at 38.7, 36, 33.5, 26.9, and 24.1 Ka that they call the
Campanian Ignimbrite series. While we do not dispute that there are
five marine tephra layers that may be genetically related to one
another and to the ignimbrite on land, we prefer to include them under
the name "Campanian eruptive events." There appears to be only one
major ignimbrite -- the Campanian Ignimbrite -- associated with the
eruptive events. According to Paterne et al. (1988), the layer dated
at 33.5 Ka corresponds to the eruption that produced the deposit
called the Campanian Ignimbrite. C14 dates (Capaldi et al., 1985;
Alessio et al., 1971, 1973, 1974) and K/Ar dates (Curtiss, 1966;
Cassignol and Gillot, 1982) give a scatter of dates between 25 and 42
Ka for the Campanian eruptive events.

The Campanian Ignimbrite is a gray, poorly to moderately welded,
trachytic ignimbrite sheet. It consists of pumice and lithic fragments
in a devitrified matrix that contains sanidine, lesser plagioclase
rimmed by sanidine, two clinopyroxenes, biotite, and magnetite.
Pumice clasts ranging from trachyte to alkali trachyte generally
increase in size upward, and lithic fragments tend to increase in size
and abundance downward in individual sections. Pumice fragments are
invariably rounded. Regionally, pumice and lithic fragments decrease
in size away from the Bay of Naples region (Barberi et al., 1978).

The Campanian Ignimbrite is thickest beneath the plain crossed by
the lower reaches of the Volturno river and in the many valleys that
drain surrounding fault-block limestone mountains. It crops out in
valleys on both sides of 1000+ m high mountain ridges to the north,
east and south of Naples. East of Naples it lies within the rugged
terrain of the Appenine Mountains; northward across the Volturno Plain
the ignimbrite crops out on the sides and within the crater of
Roccamonfina Volcano. To the south the pyroclastic current crossed
the Bay of Naples (presently as deep as 200 m) and encountered the
Sorrento Peninsula (650 to >1000 m elevation), leaving thick deposits
on the north and south sides of the peninsula. To the west is the
Tyrrhenian Sea, where sampling was not done. The ignimbrite is
exposed as far as 60 to 70 km from Pozzuoli Bay, deposits at
the most distal exposures being up to 5 m thick. We therefore
consider that the original pyroclastic current extended much farther,
possibly 100 km. The extent of the Campanian Ignimbrite, though
dispersed in isolated outcrops because of ponding during flow and
later extensive erosion, suggests that the original pyroclastic
current flowed across some 30,000 km2 of area, and laid down a deposit
estimated to be about 500 km3 in volume (bulk volume, not DRE). This
was crudely estimated by circumscribing a circle of deposits with a
radius of 100 km, 100 m thick at the center that become zero at the
perimeter of the circle. It was the largest eruption of the last
200,000 years in the Mediterranean region (Barberi et al., 1978).

DISTAL FACIES OF THE CAMPANIAN IGNIMBRITE

Glass shards of the Campanian Ignimbrite are unique;
long, straight to slightly curved plates that were derived from a
highly vesiculated magma that consisted of large (centimeters)
vesicles stretched into pita-bread-like pockets. Some shards are
semicircular remnants of large spherical vesicles. The magma just
before eruption is estimated to have exceeded 80% vesicles by volume.
Pumice clasts have more poorly-developed vesicles, ranging in volume
from 37% to 62%. This mixture of ash and pumice implies that the
vesiculated magma was a bimodal mixture of highly-inflated vesicles
and patches with poorly developed vesicles and lower overall
vesicularity. Most of the deposit consists of the distinctive
plate-like shards.

The stratigraphic sequence of the Campanian Ignimbrite in medial
and distal areas can be divided into two parts based upon the
idealized ignimbrite defined by Sparks et al. (1973); a very
thin (1 - 10 cm), discontinuous, fines-poor layer called layer 1,
above which lies the bulk of the ignimbrite called layer 2. Layer 2
is divided into layer 2a, the finer grained basal zone, and 2b, which
forms the bulk of the deposit. Layer 3 is not preserved in the
Campanian Ignimbrite sequence. The Campanian Ignimbrite is easily
correlatable from place to place because it commonly is a nonwelded
tuff with a characteristic blue-gray color, contains identifiable
sanidine, pyroxene and some biotite, and it rests on soils developed
upon older volcanic rocks or limestone. In many areas the tuff has
been diagenetically altered to a yellowish color.

The highest elevation reported for the Campanian Ignimbrite is
1000 m above sea level on top of a dome in the crater of Roccamonfina
Volcano (Giannetti, 1979), 60 km north of the center of the Bay of
Pozzuoli. At one place 55 km due east of the Bay of Pozzuoli, the
Campanian Ignimbrite lies at 970 m above sea level.

Layer 1.

Layer 1 is composed predominantly of phenocrysts
of clinopyroxene, sanidine and plagioclase, and lithic clasts
consisting of altered, poorly sorted brown tuffs, trachytic lavas and
hornfelsed lavas. The layer commonly rests on limestone soil or
limestone talus. In some areas, notably Roccamonfina Volcano, it
rests upon older volcanic rocks. In some outcrops to the east and
southeast of Pozzuoli Bay, the ignimbrite lies upon a co-genetic
Plinian fallout pumice layer as thick as 100 cm. Layer 1 is mixed
with material from the underlying terrains -- volcanic lapilli where
it lies above Plinian fallout, and limestone lapilli where it lies
above limestone basement.

Layer 1 has been identified throughout the distal region, from
Roccamonfina Volcano 40 - 70 km north of Pozzuoli Bay, to Salerno 65
km to the south. In all exposures showing the base of the Campanian
Ignimbrite, layer 1 is a lenticular to continuous layer as thick as 10
cm. In its thickest occurrences it is cross bedded showing flow
directions downslope irrespective of source direction. Layer 1
consists mostly of sand-size lithic fragments and crystals, is devoid
of medium to very fine ash, and in some places contains lithic lapilli
to 9.5 cm. In places, it occurs in pockets or in lenticular mounds
above a flat surface that was sheared and eroded during emplacement of
the overlying layer 2.

Layer 2.

At some localities, as far north as Mondragone and as
far south as Salerno, layer 1 is overlain by 10 to 20 cm of
fine-grained and generally pale gray to yellowish ignimbrite (layer
2a). This is followed transitionally upward by coarser grained and
darker gray layer 2b ignimbrite, the uppermost part commonly being a
yellow color caused by diagenetic alteration.

Textural discontinuities at some localities give a crude bedding
that is internal within otherwise homogeneous-appearing ignimbrite.
The crude bedding features range from distinct textural breaks to
extremely subtle color changes that are discontinuous across exposures
of 10 to 20 meters in length. Because the bedding are not continuous,
we suggest that the bedding was formed from overlapping of lobes of
the pyroclastic current that formed by drainage of flows off mountain
slopes from different directions. We favor this possibility because
the best examples of these textural discontinuities are in the lee of
high (>1000 m) mountain ranges. A similar phenomenon is reported from
blast surge deposits of the May 18, 1980 eruption of Mount St. Helens
(Fisher, 1990). Such crude bedding features may also be caused by
unsteady flow (Branney and Kokelaar, 1992).

Determination of flow directions in ignimbrites is commonly
difficult because the fabric in these deposits is visually nearly
isotropic. In some cases flow directions have been successfully
determined from textural indicators. These include use of imbricated
logs (Froggatt et al., 1981), as well as orientation of glass shards,
crystals, pumice, and lithic fragments (Potter and Oberthal, 1987;
Elston and Smith, 1970; Ui et al., 1989; Suzuki and Ui, 1982, 1983,
1988; Buesch, 1991; Seaman and Williams, 1992). Imbrication of pumice
and other features indicate not only a flow azimuth (bidirectional)
but the flow direction as well (Kamata and Mimura, 1983; Suzuki and
Ui, 1988). These methods, however, are time consuming and the
features are commonly absent in specific outcrops.

Determination of anisotropy of magnetic susceptibility (AMS) is
another method for determining lineations and foliations in
ignimbrites. This method detects the alignment of magnetic minerals
in the ignimbrite. The shape anisotropies cause the ignimbrite to be
variably susceptible to the acquisition of an induced magnetic moment,
forming a susceptibility ellipsoid with axes K1, K2, and K3. The K1
axis, the axis of greatest susceptibility, is generally interpreted as
the axis in which the flow lineation lies. The K3 axis is commonly
perpendicular to the plane of foliation.

Several studies have shown that the K1 susceptibility axis lies
in the direction that the flow moved (e.g. Ellwood, 1982; Incoronato
et al., 1983; Knight et al., 1986; Wolff et al., 1989; MacDonald and
Palmer, 1990; Palmer, et al., 1991; Hillhouse and Wells, 1991). These
studies have confirmed that AMS can determine flow directions that
agree with physical flow textures such as imbricated clasts. In
addition, Ellwood (1982) demonstrated that magnetic lineations in
ignimbrites of known origins are oriented radially away from those
sources, as would be expected. The method is relatively fast and
therefore can include many sites, and relatively weak lineations in
ignimbrites can be determined with a high degree of precision.

Paleomagnetic analyses of the Campanian Ignimbrite were made on a
Kappabridge KLY-2 susceptibility bridge. Results from 25 sites were
plotted and Bingham statistics for each site were determined using the
Stereonet program. Results are presented as eigenvectors,
including their azimuth and plunge, and eigenvalues, in which values
closer to unity indicate better clustering of the AMS data.
Eigenvectors may give a sense of the direction of flow. Previous
studies (Knight et al., 1986; Wolff et al., 1989; MacDonald and
Palmer, 1990) have noted an imbrication of AMS directions, such that
the vectors dip up-flow. At each sampling
site where the base is exposed, the substrate angles were measured in
order to determine imbrication directions. Where the base is
unexposed, substrate angles were estimated.

Sample localities were generally chosen to investigate the
interaction of topography with the Campanian pyroclastic current.
These include (1) the base of slopes facing the probable source
direction of the current as well as in the lee of slopes, (2) in areas
of enclosed drainage with single canyon outlets, and (3) on valley
sides along drainage systems.

The AMS trends are nearly always parallel to local stream valleys
or slope directions, and commonly at high angles
to the radial direction from any reasonable source area. In addition,
most AMS plunge directions point up slopes, implying that they reflect
imbrication and provide information on flow directions. Some of the
measured flow directions point toward the source area. This is
counter intuitive, because it is commonly assumed that pyroclastic
currents move away from their source, therefore flow directions
measured in the ignimbrite should also be away from source. How does
a pyroclastic current bypass an area and then return from the opposite
direction, particularly if this occurs on successive slopes at
increasing distance from the source? The explanation of this apparent
paradox has profound implications for determining flow mechanisms of
large volume ignimbrites that occur on both sides of mountain ranges
and within intermontane basins.

Six of the sites used to measure flow directions of the Campanian
Ignimbrite were sampled in the 300 Ka Roccamonfina Volcano area
because the presence of this edifice and its crater would have
affected the directions of movement of the pyroclastic current and
provide an enclosed basin, which in this case has a single valley
outlet. AMS eigenvectors for 4 samples have directions indicating radial
flow off the volcano. The direction of flow indicated by another sample is
east-west, parallel to the valley that drained the volcano, rather
than northward from the source of the pyroclastic flow. This suggests
that much of the material flowed radially off the outer slopes of the
volcano irrespective of the direction from the original source area.
Within the crater, one locality shows that the flow moved off the
dome into the caldera. This is the same dome on which Campanian
Ignimbrite was reported at an elevation of 1000 m.

Three sites in the Volturno Plain were measured where there would have
been no topographic interference with the
movement of the pyroclastic current. The eigenvectors from these
sites suggest radial flow outward from an area in the vicinity of the
Phlegrean Fields. These results do not locate a source precisely, but
they strongly indicate a source south of the Volturno Plain near or
within the Bay of Pozzuoli.

A major stumbling block in interpreting AMS directions is that
the sites of magnetism in ignimbrites and the causes of imbrication
(plunge) of eigenvectors is not known. Therefore the length of
movement over which the magnetic orientation is set within the
ignimbrite cannot be estimated. It may occur over a distance of
centimeters in the very last stages of movement and therefore be
imprinted by minor creep on steep slopes. However, many sites were
sampled in open valley drainage systems where the underlying surface
is nearly horizontal. Samples OF37, OF38, OF39, and OF40 (Fig. 4),
for example, exhibit strong alignment of the AMS directions that
correspond to the valley trend. This suggests that the AMS
eigenvector is a measure of the direction that the ignimbrite was
moving at the time of deposition, and not due to downvalley creep.
Most other AMS sites also show a strong correlation between
topographic orientations and flow directions. At Acqua Fidia, 970 m
above sea level, the flow direction is down and parallel to the slope,
suggesting that the flow may have drained from an area higher up the
mountain.

DISCUSSION

The distribution of the Campanian Ignimbrite together with AMS
measurements indicates that the Campanian pyroclastic flow surmounted
ridges higher than 1000 m to enter some intermontane basins that have
restricted external drainage through single narrow valleys. AMS data
indicate that the Campanian flow moved down drainage systems
originating in the intermontane basins. It did not move up the
valleys into the basins. For example, Tocco Caudio, an abandoned
village situated upon 15 m of Campanian Ignimbrite 55 km from source,
lies in the lee of limestone ridges with elevations exceeding 1000 m.
Another example is the watershed in which Cusano village is located 66
km northeast of the Bay of Pozzuoli. To gain access to the watershed,
the pyroclastic flow had to (1) surmount ridges higher than 1000 m, or
else (2) flow up a narrow, winding, deeply incised canyon before
spreading out within the watershed. There is no Campanian Ignimbrite
within the narrow confines of the canyon, but AMS data from the
ignimbrite within the watershed shows down-valley current directions.
We hold that the above relationships are best explained by the EF
model.

An explanation of the apparent contradiction that AMS data do not
show a correlation between measured flow directions and the direction
to the source is aided by application of the idea of a transport
system that differs from the deposition system as proposed for blast
surge deposits at Mount St. Helens (Fisher, 1990). At Mount St.
Helens the outward flowing pyroclastic current was the transport
system that carried fragments to accumulation sites, and rapidly
accumulating material at the base of the transport system formed the
deposition system that had the ability to flow before coming to a complete
rest. Geological evidence (Fisher et al., 1987; Brantley
and Waitt, 1988; Fisher 1990; Druitt, 1992) indicates that the
deposits from the expanded cloud decoupled from the transport system,
moved gravitationally in the directions of existing slopes, and
drained down some valleys as secondary pyroclastic flows. Photographs
of the initial 1980 eruption at Mount St. Helens (Lipman and
Mullineaux, 1981) indicate that the pyroclastic current (blast surge)
extended as a turbulent cloud several hundred meters higher than the
topography.

As indicated at Mount St. Helens, the transport of debris in an
expanded flow with consequent deposition can continue across and in
the lee of high ridges with possible generation of basal high-density
sediment gravity flows draining into every watershed. Blocking of the
flows occurs only in their basal parts where density values rise to
such high levels that they cannot surmount the mountains (Valentine,
1987; Fisher, 1990).

In the case of the plug flow model (Sparks, 1976), a dense flow
moving across the landscape under its own momentum may deposit all of
its materials as a continuous sheet before any post-flow downhill
movement takes place. Following emplacement, deposits of the sheet
would then begin to creep or to flow downhill, imprinting the AMS flow
fabric within the last few minutes of movement. Simultaneous
emplacement of the entire sheet, however, would require that there is
no momentum loss from the front to the back of the pyroclastic flow,
and that the front of the flow comes to rest at the same time as the
back of the flow. The space problem inherent in the emplacement of
dense pyroclastic flows and the problems associated with en masse
deposition have been pointed out by Branney and Kokelaar (1992).

Even allowing that a DF could move as a modified plug flow over
steep mountainous terrain, and that deposition did not occur by
instantaneous freezing throughout its extent, it is still difficult to
explain how a high density ignimbrite sheet could be emplaced without
the development of severe damming effects on distribution. Consider,
for example, a DF moving up a steep mountain front with an average
inclination of 45 degrees or more and with 1000 m of relief. In such steep
terrain, materials being laid down by a dense flow could develop
downward counter-flow against the forward moving current. AMS data in
the Campanian Ignimbrite do not indicate opposing flow directions on
steep slopes. Flow decoupling (Fisher, 1990, 1995; Buesch,
1992) could occur, but opposite moving dense flows would tend to
create considerable drag and perhaps complex mixing at their
interface, greatly reduce momentum on source-facing slopes, and
restrict distribution of the deposits. This condition would be
compounded if more than one ridge were encountered along the path of
flow as is the case in the Campanian Ignimbrite distribution area.
Furthermore, if it were possible for sheet-like dense flows to move
over mountainous ridges into watersheds and then drain out through
stream valleys, they could be blocked by another part of the same
dense current moving up the valley.

Downslope gravity-driven turbidity currents, subaqueous analogues
of pyroclastic flows and surges, exhibit behavior similar to that
described herein for pyroclastic currents. "Upslope flow" is a term
applied to observations that turbidity currents with enough momentum
and/or flow thickness, can deposit sand and silt hundreds of meters up
slopes and across bathymetric barriers (Shipley, 1978; Damuth, 1979;
Damuth and Embley, 1979; Moore et al., 1982; Cita et al., 1984;
Underwood, 1987; Dolan et al., 1989). Pickering et al. (1992)
measured paleocurrent directions from a modern deep-marine basin,
giving evidence that turbidity currents can be deflected or reflected
at submarine barriers. From laboratory experiments, Muck and
Underwood (1990) conclude that the most important variable influencing
upslope movement of a turbidity current is the flow thickness.

The idea of reflection of deflected turbidity currents was used
by van Andel and Komar (1969) to explain ponded sediments in marine
basins. There have been some studies of flow reversals in turbidity
current deposits (Ricci Lucchi and Valmori, 1980; Pickering and
Hiscott, 1985), and they indicate that reverse-flow deposits can
immediately follow those deposited by the primary flow. Such
sequences are believed by researchers to be flow reversals caused by
reflection rather than by later independent currents from the opposite
direction. Reflection has been experimentally reproduced in flumes
(Pantin and Leeder, 1987).

A key circumstantial relation suggesting that the Campanian
pyroclastic transport system was an expanded flow is the fact that the
Campanian Ignimbrite is over 43 m thick on the north side of the
Sorrento Peninsula, some 35 km from Pozzuoli Bay across the Bay of Naples.
Moreover, Campanian Ignimbrite up to 15 m thick occurs within the watershed
draining into the sea at Maori on the south side of the Sorrento Peninsula.
It is virtually impossible for the Campanian pyroclastic flow to have flowed
around the Sorrento Peninsula to enter the Maori watershed from the south,
therefore it must have overtopped the peninsula and flowed down the
valleys. The lowest pass on the crest of the watershed is at 685 m
altitude. At the time of the emplacement of the Campanian Ignimbrite,
the Gulf of Napoli existed. Sea level may have been close to
its present level between about 35,000 and 25,000 years ago and then
receded as the last full glacial episode began (Kennett, 1982, p.
269). If sea level during the eruption of the Campanian Ignimbrite
were nearly the same as today, the pyroclastic flow would have had to
flow 25 to 35 km across open water to reach the Sorrento Peninsula.
If the current were a dense pyroclastic flow, it would have probably
entered the water, as has occurred in some historic pyroclastic
currents (e.g., 1815 Tambora, Sigurdsson and Carey, 1989; 1902 Mt.
Pele, Lacroix, 1904; 1976 Augustine Volcano, Fisher and Schmincke,
1984). At Mont Pelee the dense part of the nuee ardente entered the water
while the dilute cloud above it flowed across the water, capsizing
some of the boats and setting others on fire.

At Krakatau in 1883 a pyroclastic flow entered the sea, greatly
altered the bathymetry around the volcano and generated tsunamis
responsible for the deaths of many of the 36,000+ people (Self and
Rampino, 1981; Sigurdsson et al., 1991). Sigurdsson et al. (1991)
concluded that the initial phase of the Krakatau pyroclastic current
was less dense than water, but after partial runout the flow base
became denser than sea water by gravitational segregation, a process
postulated by Fisher and Heiken (1982) for the 1902 nuee ardente at
Mont Pelee. The upper part of the Krakatau pyroclastic current
remained less dense than seawater and traveled >50 km over open water
to the Sumatra coast where >2000 people lost their lives by burns.
According to Sigurdsson et al. (1991), at 20 km from source, the
current was still expanded to 800 m in height.

Because of profound mixing conditions that would occur between a
submerged pyroclastic current and ambient water, it is not possible
for the entire Campanian pyroclastic current to have entered and moved
under water for 25-35 km to re-emerge as a hot, gas-inflated sediment
gravity flow, and then move over the 650-1000 m Sorrento Peninsula to
deposit more ignimbrite on the other side. It is more likely that a
significant portion of it moved above the waters of the Bay of Naples
as an expanded, turbulent suspension, similar to that postulated by
Sigurdsson et al. (1991) for the 1883 Krakatau pyroclastic flow. The
expanded transport system surmounted the Sorrento Peninsula and
drained down its south and north slopes. At present there is no
information available to us that indicates if Campanian Ignimbrite
lies at the bottom of the Bay of Naples.

To conclude, our study indicates that the Campanian Ignimbrite
was emplaced by an expanded turbulent pyroclastic current. The
transport system is assumed to have been thicker than the highest
topography and moved outward at 360 degrees from the possible source in the
Bay of Pozzuoli, and the depositional system drained off ridges and
down valleys in directions dictated by slope direction.

ACKNOWLEDGMENTS

We thank Peter Kokelaar, Nancy Riggs, Richard Waitt and an
anonymous reviewer for their critical reviews, and express gratitude
for research funds provided by the National Geographic Society to RVF
in 1988 and the National Science U.S. - Italy Cooperative Research;
National Science Foundation, Grant # INT - 9003232, to RVF and GH; and
a grant from CNR, Italy to GO.

Brantley, S.R. and Waitt, R.B., 1988. Interrelations among
pyroclastic surge, pyroclastic flow, and lahars in Smith Creek valley
during the first few minutes of 18 May 1980 eruption of Mount St.
Helens, USA. Bull. Volcanol. 50:304-326.